Question

what are oncogenes? how were tumor retroviruses used to discover oncogenes? what are tumor suppressors? what are the differences between tumor suppressors and oncogenes? how does p53 function to induce cell cycle arrest and apoptosis in response to stress?

Answer 1

An oncogene is a gene that has the potential to cause cancer. In tumer cells, these genes are often mutated or expressed at high levels.

The first oncogenes were discovered through the study of retroviruses, RNA tumor viruses whose genomes are reverse-transcribed into DNA in infected animals cells. During the course of infection, retroviral DNA is inserted into the chromosomes of host cells.

Tumor suppressors genes are normal genes that slow down cell division, repair DNA mistakes or tell cells when to die this process is known as apoptosis or programmed cell death. When tumor suppressor genes don't work properly cells can grow out of control which can lead to cancer.

An important difference between oncogenes and tumor suppressor genes is that oncogenes result from the activation of proto-oncogenes, but tumor suppressor genes cause cancer when they are inactivated.

An oncogene is abnormally activated to cause cancer, while a tumor-suppressor gene is inactivated to cause cancer.It can be altered by mutation to become an oncogene and thereby cause cancer

When initiated during the cellular stress response, p53 activates transcription of p21, a cyclin- dependent kinase inhibitor. p21 blocks CDK-1 and 2 leading to cell cycle arrest at G1 and S phase. Since p53 counteracts cell growth and development, it is crucial that p53 function is strictly regulated.In normal cells the p53 protein level is low. DNA damage and other stress signals may trigger the increase of p53 proteins, which have three major functions : growth arrest, DNA repair and apoptosis (cell death). The growth arrest stops the progression of cell cycle, preventing replication of damage DNA.